As one of the mass spectrometric methods, a method called MS/MS analysis (also referred to as tandem analysis) has been known for conducting identification of a substance with a large molecular weight and analysis of its structure. A triple quadrupole (TQ) mass spectrometer is a typical MS/MS type mass spectrometer. FIG. 2 is a schematic configuration diagram of a generic triple quadrupole mass spectrometer disclosed in Patent Document 1 and so on.
Inside an analysis chamber 1 evacuated by an unillustrated vacuum pump, the mass spectrometer comprises an ion source 2 ionizing a sample to be analyzed, three quadrupoles 3, 5 and 6 with each composed of four rod electrodes, and a detector 7 detecting ions and outputting detection signals corresponding to the amount of the ions. A voltage composed of a DC voltage and a high frequency voltage is applied to the first-stage quadrupole 3, and due to an effect of an electric field resulting therefrom, from various kinds of ions generated by the ion source 2 only a target ion having a specific mass-to-charge ratio is selected as a precursor ion.
The second-stage quadrupole 5 is stored in a highly airtight collision cell 4. A CID gas, such as argon (Ar) gas, is introduced into this collision cell 4. After being sent from the first-stage quadrupole 3 to the second-stage quadrupole 5, the precursor ion collides with the CID gas in the collision cell 4, giving rise to fragmentation by CID to generate product ions. Since there are various modes of such fragmentation, normally, one kind of precursor ion generates plural kinds of product ions having different mass-to-charge ratios. These various kinds of product ions exit from the collision cell 4 and are introduced into the third-stage quadrupole 6. Normally, only a high frequency voltage is applied to, or a voltage formed by adding a DC bias voltage to a high frequency voltage is applied to the second-stage quadrupole 5, so that this second-stage quadrupole 5 functions as an ion guide for transporting ions to a subsequent stage while converging the ions.
Similar to the first-stage quadrupole 3, a voltage composed of a DC voltage and a high frequency voltage is applied to the third-stage quadrupole 6, and due to an effect of an electric field resulting therefrom, only product ions having a specific mass-to-charge ratio is selected in the third-stage quadrupole 6 and reaches the detector 7. By appropriately changing the DC voltage and the high frequency voltage applied to the third-stage quadrupole 6, the mass-to-charge ratio of the ions which are allowed to pass through the third-stage quadrupole 6 may be scanned (product ion scan). In this case, based on the detection signals obtained from the detector 7, an unillustrated data processing unit may create a mass spectrum (MS/MS spectrum) of the product ions generated by the fragmentation of the target ion. In addition, a precursor ion scan which scans all the precursor ions generating specific product ions, and a neutral loss scan which searches for all the precursor ions with a specific part of structure detached or the like are executable.
In addition, in devices where the aforementioned MS/MS type mass spectrometer is used as a detector of a liquid chromatograph (LC) and a gas chromatograph (GC), such as in LC/MS/MS and GC/MS/MS, a method called MRM (Multiple Reaction Monitoring) is often used for conducting a simultaneous analysis (identification and quantification) of multiple components contained in a sample. In an MRM measurement, with respect to each component, the mass-to-charge ratio of one kind or plural kinds of precursor ions selected in the first-stage quadrupole 3, and with respect to each precursor ion, the mass-to-charge ratio of one kind or plural kinds of product ions selected and measured in the third-stage quadrupole 6 are predetermined. Since the multiple components contained in the sample are temporally separated in the LC and GC in a previous stage, the aforementioned predetermined set of mass-to-charge ratios of the precursor ion and product ion may be respectively switched according to dissolution times (retention times) of each component. Thus, signal intensity of ions derived from each component may be sought with high precision and high sensitivity, and a quantitative measurement of the sample may be performed with high precision and high sensitivity.
To conduct a highly precise and highly sensitive analysis in an MRM measurement, it is important to correctly select the precursor ion and product ion with respect to each component. Here, for the precursor ion, it is fine to simply select a fixed one with respect to each component. However, since the product ion varies depending on the modes in which it is generated (fragmented) from the precursor ion, it is necessary to predetermine optimum values of various parameters (e.g. DC and AC voltages, etc. applied to each quadrupole) which influence the foregoing.
Conventionally, a selection of this optimum value is automatically preformed in the following method.
1) A setting for conducting a product ion scan analysis (hereafter referred to as “product ion scan event”) is converted into various parameters and is prepared in plurality.
2) All the product ion scan events are executed in sequence.
3) From mass spectra of results of executing all the product ion scan events, one to a plurality of m/z is selected in order of increasing intensity. On that occasion, m/z within a tolerance range (e.g. ±0.5 m/z, etc.) is treated as one m/z.
4) The parameter of the product ion scan event generating the selected m/z is made the optimum value.